Time-of-flight (TOF) systems that provide a measure of distance (Z) from the system to a target object without depending upon luminosity or brightness information obtained from the target object are known in the art. See for example U.S. Pat. No. 6,323,942 entitled CMOS-Compatible Three-Dimensional Image Sensor IC (2001), assigned to Canesta, Inc., now of Sunnyvale, Calif. TOF systems according to the '942 patent emit optical energy and determine how long it takes until at least some of that energy reflected by a target object arrives back at the system to be detected. Emitted optical energy traversing to more distant surface regions of a target object before being reflected back toward the system will define a greater TOF than if the target object were closer to the system. If the roundtrip TOF time is denoted t1, then the distance between target object and the TOF system is Z1, where Z1=t1·C/2, where C is velocity of light. Such systems can acquire both luminosity data (signal amplitude) and TOF distance, and can realize three-dimensional images of a target object in real time.
A more sophisticated TOF system is exemplified by U.S. Pat. Nos. 6,515,740 (2003) and 6,580,496 (2003) respectively Methods and Systems for CMOS-Compatible Three-Dimensional Imaging Sensing Using Quantum Efficiency Modulation, assigned to Canesta, Inc., now of Sunnyvale, Calif. FIG. 1A depicts an exemplary phase-shift detection system 100 according to the '740 or the '296 patents, a system in which TOF is determined by examining relative phase shift between transmitted light signals and signals reflected from the target object. Detection of the reflected light signals over multiple locations in the system pixel array results in measurement signals that are referred to as depth images.
Referring to FIG. 1A, TOF system 100 includes a two-dimensional array 130 of optical detectors 140, each of which has dedicated circuitry 150 for processing detection charge output by the associated detector. In a typical application, array 130 might include 100×100 pixels 230, and thus include 100×100 processing circuits 150. IC 110 also includes a microprocessor or microcontroller unit 160, memory 170 (which preferably includes random access memory or RAM and read-only memory or ROM), a high speed distributable clock 180, and various computing and input/output (I/O) circuitry 190. Among other functions, controller unit 160 may perform distance to object and object velocity calculations. With respect to processing detected TOF signals, portions of unit 180 and array 130 may be referred to herein collectively as a time measuring unit.
Under control of microprocessor 160, appropriately controlled drive waveforms are output by a generator 115 and use to control a source of optical energy 120, which is thus periodically energized and emits optical energy via lens 125 toward an object target 20. Collectively, the term “emitter unit” may be used to encompass generator 115 and optical energy source 120. Typically the optical energy is light, for example emitted by a laser diode or LED device 120. Some of the emitted optical energy will be reflected off the surface of target object 20, and will pass through an aperture field stop and lens, collectively 125, and will fall upon two-dimensional array 130 of pixel (optical energy) detectors 140 where an image is formed. In some implementations, each imaging pixel detector 140 captures time-of-flight (TOF) required for optical energy transmitted by emitter 120 to reach target object 20 and be reflected back for detection by two-dimensional sensor array 130. The optical detectors in array 130 can operate synchronously relative to optical energy from the emitter unit. If desired, such synchronous detection operation may be implemented with an electronic high speed shutter mechanism perhaps associated with lens 125. Using this TOF information, distances Z can be determined. Advantageously system 100 can be implemented on a single IC 110, without moving parts and with relatively few off-chip components.
An exciter 115 drives an optical energy emitter 120 with a preferably low power (e.g., perhaps 50 mW peak) periodic waveform, producing optical energy emissions of known frequency (perhaps 50 MHz to a few hundred MHz) for a time period known as the shutter time (perhaps 10 ms). Energy from emitter 120 and detected signals within pixel detectors 140 are synchronous to each other such that phase difference and thus distance Z can be measured for each pixel detector. The exemplary waveform in FIG. 1B is typical of an emitted S1 signal, whereas the phase-delayed signal of FIG. 1B is an exemplary return signal s2 that will be detected by TOF system 100 and processed to yield information, include distance Z to target object 20.
The optical energy detected by the two-dimensional imaging sensor array 130 will include amplitude or intensity information, denoted as “A”, as well as phase shift information, denoted as φ. As depicted in exemplary waveforms in FIGS. 1B and 1C, the phase shift information varies with distance Z and can be processed to yield Z data. For each pulse or burst of optical energy transmitted by emitter 120, a three-dimensional image of the visible portion of target object 20 is acquired, from which intensity and Z data is obtained (DATA′). As described in U.S. Pat. Nos. 6,515,740 and 6,580,496 obtain depth information Z requires acquiring at least two samples of the target object (or scene) 20 with 90° phase shift between emitted optical energy and the pixel detected signals. While two samples is a minimum figures, preferably four samples, 90° apart in phase, are acquired to permit detection error reduction due to mismatches in pixel detector performance, mismatches in associated electronic implementations, and other errors. On a per pixel detector basis, the measured four sample data are combined to produce actual Z depth information data.
Understandably, the accuracy of Z distance measurements can be affected by the accuracy of the clock timing signals coupled to exciter 115, and to the control of phase and/or shape of the signals output by emitter 120. Accurate Z measurements require that the phase of the signal output by emitter 120 be both stable and known relative to the phase (or any of the multiple phases) associated with time measuring unit 180 of system 100, otherwise, time measurement accuracy is degraded.
For example, consider a high resolution TOF system 100 as shown in FIG. 1A that emits electromagnetic (EM) radiation, which EM radiation travels about 1 cm for every 33 ps (1 picoseconds being 1×10−12 seconds). Thus, for a 1 cm change in distance Z, the roundtrip TOF interval is only 66 ps. If there were no other error sources in TOF system 100, the error budget to maintain 1 cm Z distance measurement accuracy must be within 66 ps, which is to say that relative phase/timing inaccuracies associated with emitted signal S1 and a time measurement system must be within a total of 66 ps.
In practice, for many TOF systems changes in environmental conditions can easily cause more than 66 ps of variation in the relative phase/timing of the emitted signal(s) and time measurement system. Additionally, it is also important to maintain other phase/timing relationships critical to the time measurement, such as in the timing generator and timing measurement unit, collectively 180 in FIG. 1A, to the same level of accuracy.
What is needed is an enhanced method and system to promote measurement accuracy in a TOF system, where it is understood that the TOF system may utilize time-of-flight and/or phase shift data to determine distance Z. Preferably such method and system should include a frequency control mechanism, a highly accurate multi-phase clock timing generator, pulse shaping for driver of the TOF optical source, and a feedback mechanism to dynamically adjust phase of emitted signals such that emitted signal phase is maintained substantially constant relative to that of the time measurement system. Preferably such method and system should be CMOS-compatible and preferably implementable on the same IC chip containing much of the TOF system.
The present invention provides such a method and system.